This invention relates to data communication. In particular the invention relates to data communication making use of the Bluetooth protocol.
Bluetooth is a widely used wireless communications medium which is extremely versatile and therefore can be used in a wide number of different circumstances. However, one of the issues with Bluetooth communication is that the establishment of a Bluetooth connection carries with it a significant processing overhead. The Bluetooth standard provides for one way around this problem in that two Bluetooth communication devices may be “bonded” to one another such that they may only communicate with one another. Once this bonding has taken place, communication between those two devices is simplified. This bonding process is used, for example, where wireless headphones are to communicate with a mobile telephone or other such source.
However, if there is a desire to communicate between one central or host device and a plurality of remote or slave devices, then this bonding process is of no use.
In such a circumstance, if conventional Bluetooth communication is used then there are two effects. Firstly, significant hardware must be provided at each location from which Bluetooth signals are to be sent, and secondly there is a significant time and power requirement associated with the overhead of establishing a Bluetooth connection. These factors can make conventional Bluetooth communications less than ideal in a situation where low cost transmission devices are required and perhaps only a small amount of data needs to be sent.
The present application relates to data communication mechanisms which make use of the Bluetooth protocol, but allow the transmission of data without the making of a Bluetooth ACL (Asynchronous Connection Less) connection.
To aid in understanding of the invention as described in further detail below, there follows a description of one aspect of the Bluetooth regime.
Bluetooth communication provides for ad hoc connections between peer devices. The ad hoc nature of the communication implies that the population of peer devices within range can, and often will, vary in time. This variation requires that Bluetooth devices have some means of determining at any time which devices are in the vicinity and ready to respond.
The procedure for discovering peer devices is called INQUIRY within the Bluetooth specification and devices which make themselves discoverable have to enable a mode called INQUIRY SCANNING to be able to participate in this inquiry process.
When a device needs to discover devices in the vicinity it sends out a radio packet repeatedly. This packet is known as an “ID Packet” and consists of 68 bits of information which have been chosen to provide an optimal auto-correlation coefficient. The 68 bits of information in the ID Packet has a form as shown in FIG. 1. Four trailer bits which are used in other parts of the Bluetooth regime are omitted and the 68 bits consist of a 64 bit “SYNC WORD”, giving the good auto-correlation coefficient as mentioned above, and a four bit preamble.
When an ID Packet is received by a Bluetooth device in the appropriate state, which can be described as a Bluetooth Responder, that device or responder sends a response in the form of a Frequency Hop Sequence packet (“FHS Packet”). The FHS Packet consists of 144 bits of information padded with forward error correction bits to make an overall length of 240 bits.
The 144 bits of the FHS Packet are made up of many smaller concatenated packets of information. The make up of a FHS packet is shown in FIG. 2. All of the data which are included within the FHS packet relate to properties or parameters of one or other of the Bluetooth units. The system is thus a closed system, with all of the data in the FHS packet being generated internally by the Bluetooth system.
In the present application a field of interest is that labelled “Class of Device”. This is a 24 bit field. Class of Device is described in the Bluetooth specification as a parameter used to indicate the capabilities of a device and is made up of sub-fields as shown in FIG. 3.
In the present application, the major device class sub-field which consists of 5 bits and the minor device class sub-field which consists of 6 bits are of particular interest. These bits can be used to indicate the device class of a responder. However, in some circumstances there is no need to transmit this 6 bit field, and in such a case the major device class sub-field may be set to indicate an unclassified device class. In this circumstance the 6 bits in the minor device class field are not defined in the Bluetooth specification and so can be set to anything and used for an alternative purpose.